POLYCHAETA. 



461 



curved hook (Terebelliforinia, Sabellifonnia). When the parapodia 

 are completely wanting, tlie setae project from the body -wall 

 (Capitella). 



! 



K 



ri. c d P / 



FIG. 365. Setae of different Polycltaeta (after Malmgren and Claparede) , hooked setae of 



crasaicornis ; ft, of Terebella Danit'lsseni ; c, seta with spiral ridge from Sthenelais; 

 d, lance-shaped seta of Phyilochaetopterus; e, of Subella crassieornis ; f, of Sabclla jmvonis ; 

 </, composite, sickle-shaped seta of Nereis eitltrifera. 



The appendages of the parapodia present a great variety of form, 

 and not unfrequently vary in the different parts of the body. Most 

 important are the cirri, which are attached to the dorsal and ventral 

 surfaces of the parapodia. The cirri are for the most part filiform, 

 and sometimes pointed ; 

 they may also be ringed. 

 In some cases the dorsal 

 cirri are flattened out as 

 broad scale-like structures 

 -the elytra (Fig. 366)- 

 which constitute a protec- 

 tive covering to the back 

 (Aphroditidae). In other 

 cases the dorsal cirri* are. 

 modified as branchiae, which 

 may be filiform, branched 

 and antler - like, comb - 

 shaped, or in tufts ; some- 

 times they are confined to 

 the middle region of the 

 body (Arenicola, Fig. 397), 



* It sometimes happens that cirri are found on branchiferons segments, in 

 which case the gills must l>e regarded as additional to cirri. 



El 



Fia. 366. Anterior end of Polynoe i:rti'nimlri, the first 

 elytron on the left hand being removed (after 

 Claparede). The two setae of the oral segment aie 

 visible. El elytron. 



